The top image shows monthly anomalies from five different evapotranspiration (ET) estimates and the Southern Oscillation Index (SOI) for 1980–2011. The bottom image shows, during El Niño months (with low SOI, marked by red line on top), the average monthly anomalies of ET (left) and the independent ESA-CCI soil moisture (right).

Using data from Earth-observing satellites, scientists have found that the global average evapotranspiration declines are the result of droughts in Australia and southern Africa, which are caused by El Niño conditions.

During El Niño periods, reduced precipitation in these regions leads to lower terrestrial moisture content, so there’s less water in the soil available to evaporate. These regional reductions are so intense they severely affect the global average volume of evapotranspiration. Therefore, global average evapotranspiration declines aren’t caused by a more permanent reorganization of the water cycle, but by natural climate variability.

“These El Niño-induced declines are complementary to a general, long-term trend caused by global warming,” said Diego Miralles from the University of Bristol, the study’s principal investigator. “In fact, we also uncovered an increase of evapotranspiration in regions less affected by the El Niño–La Niña cycle like the northern latitudes. There, the rates of increase agree with the expectations derived from the warming observed in recent decades.”

The study was carried out under ESA’s Water Cycle Multi-Mission Observation Strategy—Evapotranspiration project, funded by the Support to Science Element.